Appendix A, included herein as pages 54-62, is a listing of computer programs and related data for use with Visual Basic software version 5.0, 1997, available from Microsoft Corporation. The software may be loaded into a personal computer for implementing a method and apparatus as described below in reference to FIGS. 4A-4F in one illustrative embodiment of this invention.
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
In the processing of a semiconductor wafer to form integrated circuits, charged atoms or molecules are directly introduced into the wafer in a process called ion implantation. Ion implantation normally causes damage to the lattice structure of the wafer, and to remove the damage, the wafer is normally annealed at an elevated temperature, typically 600xc2x0 C. to 1100xc2x0 C. This anneal also causes implanted atoms to move from interstitial sites to substitutional sites in the crystal lattice (an atom must be in a substitutional site to be electrically active). Prior to annealing, material properties at the surface of the wafer may be measured, specifically by using the damage caused by ion implantation.
For example, U.S. Pat. No. 4,579,463 granted to Rosencwaig et al. (that is incorporated herein by reference in its entirety) describes a method for measuring a change in reflectance caused by a periodic change in temperature of a wafer""s surface (see column 1, lines 7-16). Specifically, the method uses xe2x80x9cthermal waves [that] are created by generating a periodic localized heating at a spot on the surface of a samplexe2x80x9d (column 3, lines 54-56) with xe2x80x9ca radiation probe beam . . . directed on a portion of the periodically heated area on the sample surface,xe2x80x9d and the method xe2x80x9cmeasur[es] the intensity variations of the reflected radiation probe beam resulting from the periodic heatingxe2x80x9d (column 3, lines 52-66).
As another example, U.S. Pat. No. 4,854,710 to Opsal et al. (also incorporated herein by reference in its entirety) describes a method wherein xe2x80x9cthe density variations of a diffusing electron-hole plasma are monitored to yield information about features in a semiconductorxe2x80x9d (column 1, lines 61-63). Specifically, Opsal et al. state that xe2x80x9cchanges in the index of refraction, due to the variations in plasma density, can be detected by reflecting a probe beam off the surface of the sample within the area which has been excitedxe2x80x9d (column 2, lines 23-31) as described in xe2x80x9cPicosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium,xe2x80x9d by D. H. Auston et al., Physical Review Letters, Vol. 32, No. 20, May 20, 1974. Opsal et al. further state (in column 5, lines 25-31 of U.S. Pat. No. 4,854,710): xe2x80x9cThe radiation probe will undergo changes in both intensity and phase. In the preferred embodiment, the changes in intensity, caused by changes in reflectivity of the sample, are monitored using a photodetector. It is possible to detect changes in phase through interferometric techniques or by monitoring the periodic angular deflections of the probe beam.xe2x80x9d
A brochure entitled xe2x80x9cTP-500: The next generation ion implant monitorxe2x80x9d dated April, 1996 published by Therma-Wave, Inc., 1250 Reliance Way, Fremont, Calif. 94539, describes a measurement device TP-500 that requires xe2x80x9cno post-implant processingxe2x80x9d (column 1, lines 6-7, page 2) and that xe2x80x9cmeasures lattice damagexe2x80x9d (column 2, line 32, page 2). The TP-500 includes xe2x80x9c[t]wo low-power lasers [that] provide a modulated reflectance signal that measures the subsurface damage to the silicon lattice created by implantation. As the dose increases, so does the damage and the strength of the TW signal. This non-contact technique has no harmful effect on production wafersxe2x80x9d (columns 1 and 2 on page 2). According to the brochure, TP-500 can also be used after annealing, specifically to xe2x80x9coptimize . . . system for annealing uniformity and assure good repeatabilityxe2x80x9d (see bottom of column 2, on page 4).
An apparatus and method in accordance with the invention stimulate a region of a semiconductor wafer (also called xe2x80x9csemiconductor substratexe2x80x9d) that originally has a first number of charge carriers, so that there are a second number of charge carriers during the stimulation. The stimulation can be accomplished in any number of ways, including e.g. by use of a beam of electromagnetic radiation or by a beam of electrons. The apparatus and method use a measurement device (such as an interferometer in one embodiment) to obtain a measured value of a signal that is affected by the stimulation. In one embodiment, the affected signal is a probe beam that is reflected by the charge carriers, although other signals can be used in other embodiments.
The apparatus and method also operate a simulator (e.g. a personal computer programmed with simulation software) to generate a simulated value for the measured signal. The simulated value is based on: (i) conditions present during stimulation (as described above) and (ii) a predetermined profile of the concentration of active dopants in the region under stimulation. If the measured value matches the simulated value, then the predetermined profile used in simulation is used as a measure of the profile of active dopants in the region. The simulation may be repeated with a number of such predetermined profiles.
In one implementation, the simulations are repeated (prior to the stimulation) to obtain a set of such profiles, and the corresponding simulated values are used later to obtain a measure of the profile of active dopants in the region, e.g. by finding the closest simulated value to the measured value. In another implementation, one or more simulations are repeated after the stimulation only in case there is no match, until the simulated value and the measured value differ by less than a predetermined amount (e.g. less than 1%), and the corresponding predetermined profile is used as a measure of the profile of active dopants in the region.
The measured profile of active dopants can be used in a number of ways. In one embodiment, the measured profile is used to determine junction depth that is compared with specifications for acceptability of the wafer. If the junction depth falls within the specifications, the wafer is processed further (e.g. in a wafer processing unit to form another layer on the substrate, or in an annealer for heat treatment of the substrate), and otherwise the substrate is identified as unacceptable and placed in a bin of rejected substrates.
In one embodiment, the apparatus and method creates charge carriers in a region of the semiconductor material (also called xe2x80x9ccarrier creation regionxe2x80x9d) in a concentration that changes in a periodic manner (also called xe2x80x9cmodulationxe2x80x9d) only with respect to time. Thereafter, the apparatus and method determine the number of charge carriers created in the carrier creation region by (1) measuring an interference signal obtained by interference between a reference beam and a portion of a probe beam that is reflected by the charge carriers, and (2) comparing the measurement with predetermined data (e.g. in a graph of such measurements plotted against junction depth).
Charge carriers that are created as described above (also called xe2x80x9cexcess carriersxe2x80x9d) are in excess of a number of charge carriers (also called xe2x80x9cbackground carriersxe2x80x9d) that are normally present in the semiconductor material in the absence of illumination. The concentration of excess carriers is modulated in time at a frequency that is maintained sufficiently small to ensure that the variation in concentration is aperiodic (i.e. not oscillatory, e.g. decays exponentially or according to a monotonic function). Specifically, a profile of excess carrier concentration that is devoid of a wave (along radial distance) is created as described herein when at least a majority (i.e. greater than 50%) of the charge carriers that move out of the carrier creation region do so due to diffusion.
Such a temporal modulation under diffusive conditions (also called xe2x80x9cdiffusive modulationxe2x80x9d) is used to measure an interference signal, (for example, the phase and amplitude are both measured). The measurement is used to determine (e.g. by looking up a graph or a table) one or more properties (also called xe2x80x9csemiconductor propertiesxe2x80x9d) of the semiconductor material (such as junction depth). The concentration of excess carriers as a function of depth from the front surface of the semiconductor material, when measured as described herein, can also be used to determine the concentration of active dopants in the semiconductor material. Specifically, a profile of excess carrier concentration in depth is a function of the depth profile of the electric field that results from the active dopants (that form the doped semiconductor material).
An increase in excess carriers as a function of depth causes a corresponding increase in an index of refraction of the semiconductor material. Therefore, a laser beam (called xe2x80x9cprobe beamxe2x80x9d) shone on the semiconductor material is reflected back (by both background carriers and by excess carriers, but only the reflection by the excess carriers varies periodically at the modulation frequency), and a signal at the modulation frequency generated by interference between the reflected portion and a reference beam is measured as described herein. Various properties of the interference signal (such as amplitude and phase) are functions of the depth at which the reflection occurs, and can be measured to determine the depth of the junction. Note that as used herein, a junction is the boundary of any doped region (irrespective of whether the doping is a p-type dopant into an n-type substrate, a p-type dopant into a p-type substrate, or vice-versa).
A first embodiment (also called xe2x80x9cfront surface embodimentxe2x80x9d) measures the intensity of an interference signal that is obtained by interfering the reflected portion of the probe beam with a reference beam formed by another portion of the probe beam (this portion hereinafter being called xe2x80x9cfront surface beamxe2x80x9d) that is reflected by the front surface. In one variant of the front surface embodiment, a laser is used to generate another beam (called xe2x80x9cgeneration beamxe2x80x9d) that is used to generate the excess carriers. The generation beam""s intensity is modulated at a fixed frequency that is sufficiently low to ensure that the phase of the variation of the concentration of excess carriers is the same as (e.g. to within 10%) the phase of the generation beam over a diffusion length (wherein diffusion length is the length over which the excess charge carrier concentration decays to 1/e). Therefore, the excess carrier concentration changes approximately synchronously with the change in intensity of the generation beam. This condition ensures that the excess carrier distribution is primarily due to diffusion that can be modeled by a non-wave solution (rather than by a wave solution).
In this variant, an interferometer measures the amplitude and phase of such an interference signal, as a function of the generation beam""s power and modulation, and these measurements are used to determine the concentration of excess carriers. Variation in time of the excess carrier concentration as described above allows the interferometer to use a lock-in amplifier to measure the reflected portion of the probe beam with an accuracy not possible when the excess carrier concentration is fixed.
In one implementation, a number of graphs relating the interference signal measurement to the junction depth and to the power of generation beam are determined (either by simulation or empirically). Thereafter, for a given wafer (also called xe2x80x9cproduction waferxe2x80x9d), measurements (also called xe2x80x9cinterference measurementsxe2x80x9d) of the interference signal for different powers of the generation beam are performed, and the results are compared to one or more of the just-described graphs, thereby to determine a graph that indicates the junction depth. Specifically, predetermined graphs are generated in the following manner for a number of dopant profiles that approximate an expected dopant profile of the production wafer.
Any method or device can be used to generate dopant profiles that are provided as input to a simulator (that may be a programmed computer executing a simulation program) for generation of the predetermined graphs. For example, spreading resistance profiles can be obtained on wafers (also called xe2x80x9creference wafersxe2x80x9d) that have been processed under known conditions and have known properties. Alternatively, dopant profiles can be simulated using commercially available simulators (that assume movement of charge carriers from the carrier creation region by diffusion).
Next, for a given dopant profile, a profile of the excess carrier concentration as a function of depth is determined using a simulator, for each of a number of powers of the generation beam. Next, a derivative of excess carrier profile as a function of depth z from the front surface is multiplied by cos(2 knz), wherein k=2xcfx80/xcex, with xcex being the wavelength of the probe beam, and n being the index of refraction of silicon. The product of multiplication is integrated with respect to depth and multiplied by one or more constant factors (that are related to known physical constants and to calibration of the measurement system) to determine a simulated value of the interference measurement. The simulated value of the interference measurement is thereafter plotted on a graph as a function of depth z for a selected generation beam power. The just-described acts are repeated to obtain graphs for other fixed values (e.g. two additional values) of the generation beam power. Additional such graphs are generated for different dopant profiles.
After such graphs are available, interference measurements on a production wafer at the selected generation beam power are used to look up the graphs to determine junction depth. The look up can be repeated for different interference measurements obtained by using different powers of the generation beam, to eliminate ambiguity that may result from two wafers having different junction depths but same measurements (as may occur, e.g. when changes in the two numbers being multiplied, namely (1) the derivative and (2) the cosine function (as described above) compensate for each other in the two wafers). A predetermined dopant profile having the same junction depth as that obtained by look up is thereafter used as the profile of active dopants present in the production wafer.
Measurement of the phase and amplitude of an interference signal as described herein is a significant aspect of one implementation. One or more such measurements provide a measure of a property of the semiconductor material (or a process condition) during wafer fabrication. In another implementation (also called xe2x80x9cscanning implementationxe2x80x9d), a number of such measurements are performed at different locations on a wafer (while the generation beam""s power is maintained constant). Any change in such measurements indicates a corresponding change in the concentration of active dopants (at a predetermined depth from the front surface). Therefore, such interference measurements (from which active dopant profile is determined) are preferably (but not necessarily) monitored in one variant of the invention during wafer fabrication, to control a process step (e.g. to control annealing temperature of a wafer that has been ion implanted) used in fabricating the wafer.
When the junction depth and junction profile are measured directly on the wafer undergoing fabrication (also called xe2x80x9cpatterned waferxe2x80x9d or xe2x80x9cannealed waferxe2x80x9d depending on the stage of fabrication), a measurement as described herein increases yield, as compared to an off-line measurement of a test wafer""s properties. Moreover, such a measurement avoids the prior art cost of the test wafer itself. Such measurements are performed in one embodiment after annealing a production wafer to activate the dopants, thereby to obtain a measure that is more indicative of the electrical behavior of the devices being fabricated than a property that is measured prior to annealing (as described in U.S. Pat. No. 4,854,710).
In a second embodiment (also called xe2x80x9cphase embodimentxe2x80x9d), instead of the above-described interference signal, another interference signal is generated by interference between the reflected portion of the probe beam (described above) and another reference beam (hereinafter xe2x80x9cvariable phase beamxe2x80x9d) having a phase that can be changed independent of the phase of the probe beam. A phase difference (detected using, e.g. a phase detector) between two interference signals indicates the junction depth, wherein a first interference signal is obtained by interference of (1) the variable phase beam and (2) the front surface beam that is described above as the portion of probe beam reflected by the front surface, and a second interference signal is obtained by interference of (1) the variable phase beam and (2) the reflected portion of the probe beam.
In the second embodiment, the probe beam is coherent (i.e. of single chrominance, e.g. single wavelength) in addition to being polarized, so that interference with the variable phase beam can happen. Use of a reference beam in the second embodiment that is independent of the probe beam provides an increase in sensitivity of the measurement of material properties over the first embodiment, because of increased sensitivity of a phase detector used in the second embodiment to measure the interference signal. Use of an independent reference beam also allows absolute measurement of the junction depth as a fraction of the wavelength in the semiconductor.